The good news based on solid research is that you can eat more – probably more food than you're now eating – and weigh less, if you choose more of the right kinds of foods.

At a fall meeting on the worldwide obesity epidemic, Dr. Barbara J. Rolls, a professor of behavioral health at Penn State, began her presentation on weight control with this irrefutable statement:

"Calories count, no matter what you read in the press. The laws of thermodynamics have not been reversed."

With respect to weight gain and loss, the laws of thermodynamics can be translated: Calories consumed must be used or they will be stored as body fat. The body does not waste energy, no matter what its source. When people are placed on carefully controlled diets, the amount of fat in the diet has little effect on weight loss, Dr. Rolls reported.

People who claim that they can eat as much as they want (of protein and fat, for example) and lose weight as long as they avoid certain kinds of foods (carbohydrates, for example) are really eating less (that is, fewer calories) than they did before.

But what about a majority of people concerned about weight control who are not interested in cutting out breads, cereals, grapes, bananas, watermelon, carrots, beets, potatoes, rice and pasta? Are they doomed to remain hopelessly overweight?

Not according to Dr. Rolls, an expert on satiety and satiation, words that refer to what and how much a person has to eat at a meal to feel satisfied and stop eating. So what makes your body say you've eaten enough? Dr. Rolls' studies have clearly demonstrated an overriding influence of food volume, prompting her to write The Volumetrics Weight-Control Plan: Feel Full on Fewer Calories (HarperCollins, 2000) with Robert A. Barnett.

She found that what makes a big difference in how many calories people consume at a given meal, and throughout the day, is the amount of calories in a given volume of food. In nutritional parlance, this is called the energy density of the food. The greater the energy density – the more calories packed into a given amount of food – the easier it is to overeat.

"People tend to eat a consistent weight of food," Dr. Rolls has found. When consuming a calorie-dense food high in fat, people are likely to eat more calories just to get in a satisfying amount of food.

What increases food volume without adding calories? You guessed it: water. And what foods naturally contain the most water? Fruits and vegetables.

"People given the message to eat more fruits and vegetables lost significantly more weight than those told to eat less fat," Dr. Rolls said. "Advice to eat more is a lot more effective than advice to eat less."

"Water has the biggest impact on the amount of food we eat," she told an international symposium on obesity in Minneapolis. "Foods with a low moisture content increase the number of calories people eat."

What would fill you up faster – a quarter cup of raisins or one and three-quarters cups of grapes? Raisins are simply dried grapes; both contain the same macronutrients and supply the same 110 calories. The difference lies in volume – the amount of water they contain. If you ate a 475-calorie meal of soup, vegetables and fruit, you'd consume twice as much food by volume than if you chose drier, higher fat foods.

In her studies, people ate a constant weight of food, but if water contributed significantly to the weight and volume of the food, they ate about a third fewer calories. In one study, Dr. Rolls and colleagues tested the amount people ate when offered a 270-calorie chicken-and-rice casserole with a glass of water to drink, as opposed to the same ingredients prepared as a soup. The soup eaters spontaneously consumed 100 fewer calories, she reported.

In other studies, when participants were given a water-rich first course – soup or a salad, for example – before their main dish, they ate significantly fewer total calories than they did if the main course was given without the low-energy-density appetizer. In addition, study participants given foods containing lots of water and fiber ate less throughout the day.

Thus, by decreasing the energy density of foods, people naturally eat less, not just at an individual meal but all day long, Dr. Rolls reported. This was true of lean and obese participants in the study.

After water, which has no calories, fiber contributes the most to food volume for the fewest number of calories. Fiber supplies 1.5 to 2.5 calories per gram, far fewer than fat, at 9 calories, or protein and carbohydrates, at 4 calories per gram. Also, fiber holds water in the digestive tract, which contributes to a more lasting sense of fullness.

Fiber is found only in plant foods: fruits, vegetables and grains, especially whole grains. Along with water, it acts as a digestive tract stimulant; cutting out fiber-rich foods can lead to chronic constipation.

Dr. Rolls' work helps to explain why so many people who reduced the fat in their diets failed to lose weight and perhaps even gained.

People eat less when there are fewer calories in a given volume of food. If you cut back on fat and replace it with energy-dense (that is, dry) carbohydrates such as pretzels, crackers or sweets, you may not be reducing your caloric intake. You may, in fact, be increasing it.

In the winter I would have a bowl of vege soup before my main meal which consisted of lean meat and veges and I found this really fulled me up. Perhaps the summer equivalent of this is to have a light salad before a lean meat and vege meal.

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